This invention relates to magnesium sheet, and more particularly to an apparatus and method for producing magnesium sheet by roll milling.
The demand for personal electronics, fuel efficient light weight vehicles and other consumer products has driven the demand for competitively priced lightweight materials with a high specific strength and specific stiffness. In recent years magnesium alloy die castings have successfully been used in many applications, but further weight reductions have required the use of wrought magnesium sheet.
Magnesium is a metal with a Hexagonal Close Packed (HCP) crystal structure that has very limited plasticity at room temperature. Until recently, all magnesium sheet was made by hot rolling small ingots and the costs associated with the reheating operation to maintain the metal at rolling temperatures and the small coil sizes made the final sheet prohibitively expensive for consumer applications. In the case of magnesium and magnesium sheet alloys, the HCP crystal structure of the metal limits its deformation abilities at lower temperatures. This required frequent reheating in off-line ovens to maintain the temperature between 250° C. and 450° C. Below this temperature, the metal had a tendency to crack during rolling. Handling and reheating oven constraints limited the maximum slab size and traditionally made magnesium sheet production virtually a sheet-by-sheet operation. This was a very labor and energy intensive, inefficient method of production and contributed to the high cost of magnesium sheet.
Recent advances in twin rolling casting have allowed magnesium alloys to be directly cast into coils of material that are in the range of 4 mm to 7 mm thick, however only small coils of rolled magnesium sheet are available. Conventional rolling processes can only produce small coil sizes because as the ingot is rolled, it gets longer and thinner, which increases the surface area, and therefore loses heat rapidly and gets too cool to roll any further. It is not economical to off-line reheat long sections of rolled slab. Consequently a need exists for a magnesium rolling mill which provides for an industrial rolling process that not only economically reduces the cast coils to the final gauge required by the consumer products, but also has the ability to modify the microstructure of the as-cast magnesium to improve the formability of the rolled sheet, while maintaining a good quality surface that requires minimal treatment after rolling.